Recently, the general trends in designing portable electronic devices are toward small size, light weightiness and easy portability. The portable electronic devices such as mobile phones, personal digital assistants (PDAs), digital still cameras, digital video cameras, notebook computers and the like have built-in batteries. If no external power supply apparatus is provided to power the portable electronic device, the built-in battery is usually used as the main power source. If the power supplied from the battery is insufficient, the user needs to charge the built-in battery.
FIG. 1 is a schematic circuit block diagram of a conventional charging circuit. The charging circuit 1 of FIG. 1 principally includes an AC-to-DC converting circuit 11, a DC-to-DC converting circuit 12 and a filter capacitor Cbus. The AC-to-DC converting circuit 11 is electrically connected to the DC-to-DC converting circuit 12 and the filter capacitor Cbus. The DC-to-DC converting circuit 12 is electrically connected to a charger. An input AC voltage Vin is received and converted by the AC-to-DC converting circuit 11 into a high DC voltage. The noise contained in the high DC voltage is filtered off by the filter capacitor Cbus, thereby creating a first DC voltage Vbus. The first DC voltage Vbus is then converted by the DC-to-DC converting circuit 12 into a regulated DC voltage required for charging the battery 13.
In the conventional charging circuit 1, the battery is charged by a constant current. In other words, the current Ib outputted from the DC-to-DC converting circuit 12 is substantially constant in order to continuously and stably charge the battery 13. As the charge capacity of the battery 13 is increased, however, the voltage difference Vb between both terminals of the battery 13 is increased. If the current Ib outputted from the DC-to-DC converting circuit 12 continuously and stably charge the battery 13, the voltage difference Vb between both terminals of the battery 13 is continuously increased.
Furthermore, the first DC voltage Vbus outputted from the AC-to-DC converting circuit 11 is usually constant. By the DC-to-DC converting circuit 12, the first DC voltage Vbus is converted into the regulated second DC voltage, which is equal to the voltage difference Vb between both terminals of the battery 13. As a consequence, the magnitude of the second DC voltage is changed as the charge capacity of the battery 13. Generally, the relation between the first DC voltage Vbus and the second DC voltage Vb can be written as a formula: Vb=Vbus×D×N, where D is a duty cycle and N is a turn ratio. Since the first DC voltage Vbus and the turn ratio N in the above formula are constant values, the second DC voltage Vb is in direct proportion to the duty cycle D. In a case that the battery 13 has the minimum charge capacity, the voltage difference between both terminals of the battery 13 is minimum and thus the second DC voltage Vb and the duty cycle D are minimum. Whereas, in a case that the battery 13 has the maximum charge capacity, the voltage difference between both terminals of the battery 13 is maximum and thus the second DC voltage Vb and the duty cycle D are maximum.
Generally, the operating efficiency of the DC-to-DC converting circuit 12 is dependent on the duty cycle D. If the DC-to-DC converting circuit 12 is operated at a high duty cycle D, the operating efficiency is relatively higher. Whereas, if the DC-to-DC converting circuit 12 is operated at a low duty cycle D, the operating efficiency is relatively lower. Under this circumstance, the operating efficiency of the DC-to-DC converting circuit 12 is dependent on the charge capacity of the battery 13. That is, the charging circuit 1 has a low operating efficiency when the battery 13 has low charge capacity but a high operating efficiency when the battery 13 has high charge capacity. On the whole, the operating efficiency of the charging circuit 1 is unsatisfactory.
Therefore, there is a need of providing a high efficiency charging circuit so as to obviate the drawbacks encountered from the prior art.